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Barry's Coilgun Design What is a coilgun or gauss gun? It accelerates a piece of iron or steel down a tube. The tube runs through a series of electromagnetic coils (like solenoids).

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Barry's Coilgun Design Magnetic Materials Solenoid Physics Magnetic Field Force From Magnetism ＊ Force Is the Gradient of Potential Energy ＊ The potential energy in a magnetic field is Saturation ＊ The B-H curve here illustrates the effect of magnetic saturation. It shows the effect of applying an external magnetic field to unmagnetized iron. Introduction

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Barry's Coilgun Design Introduction Capacitors Energy Storage ＊ The charge or quantity of electricity that can be held in the electric field between the capacitor plates is proportional to the applied voltage and to the capacitance of the capacitor: Q = C * V where Q = charge in coloumbs C = capacitance in farads V = voltage in volts ＊ The energy stored in a capacitor is also a function of voltage and capacitance: W = V 2 * C / 2 where W = energy in joules (watt-seconds) V = voltage in volts C = capacitance in farads

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Barry's Coilgun Design Inductors (Coilgun) Inductors ＊ The symbol and defining equation for an inductor is,where L is called the inductance. Damped Oscillator (RLC) Damped Oscillator ＊ The voltage V and current I as a function of time Introduction

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Barry's Coilgun Design Critical Damping ＊ When R 2 C 2 -4LC is positive, then α and β are real numbers and the oscillator is over-damped. The circuit does not show oscillation. ＊ When R 2 C 2 -4LC is negative, then α and β are imaginary numbers and the oscillations are under-damped. The circuit responds with a sine wave in an exponential decay envelope. ＊ When R 2 C 2 -4LC is zero, then α and β are zero and oscillations are critically damped. The circuit response shows a narrow peak followed by an exponential decay. Introduction

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Barry's Coilgun Design Measuring Coilgun Speed ＊ Horizontal Ballistic Speed Trap If you fire the coilgun horizontally off a table, and measure the distance to where it lands, and the height it fell, then you have enough information to calculate the speed. ＊ speed = d * SQRT(g / 2h) 1 where d is horizontal distance in feet (or meters) and h is vertical distance in feet (or meters) and SQRT is the square root function Introduction

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Barry's Coilgun Design Barry's Coilgun (1) Result: Position ＊ The exit speed is quite sensitive to the projectile's precise starting position. It takes only a few millimeters further in or out to gain or loose significant speed. This graph shows the measured speed compared to how far the projectile was inserted into the coil. Dist (x) Muzzle Speed 18.6mm 4.46 m/s m/s

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Barry's Coilgun Design Result: Turns ＊ The timing is entirely controlled by the inductance and capacitance. The coil should be wound with taps at various layers, so you can choose the number of turns and therefore the inductance. Barry's Coilgun (1) LayersResult 14strong snap to middle of coil 12strong snap to middle of coil 10strong snap to middle of coil 8strong snap, fell out wrong end m/s forward m/s m/s

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Barry's Coilgun Design Results - External Iron ＊ This coilgun was built with iron washers at each end to help focus the magnetic field. Conclusions This coilgun works best with no external iron. Barry's Coilgun (1) With Iron Without Iron 4.78 m/s 5.93 m/s

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Barry's Coilgun Design Result: Tube ＊ Does the material of the firing tube have any effect? ＊ In running this test, I discovered the plastic firing tube made a dramatic 24% improvement in exit speed. This tells us the eddy currents are very significant! We can expect the energy losses in eddy currents to get much higher as we move to shorter firing times, since eddy currents increase with frequency. Conclusions ＊ This coilgun worked 24% better with a plastic non-conductive firing tube. This was the single biggest performance gain of any changes I've tried! The plastic tube was inexpensive, but other materials and construction techniques should also produce the same benefit. Barry's Coilgun (1) Brass Tube Plastic Tube 4.78 m/s 5.93 m/s

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Barry's Coilgun Design Result: Tube ＊ Eddy Currents The large and rapid flux changes will induce surface currents in the conductive projectile. Eddy currents always act against the applied magnetic field, reducing the absorbed kinetic energy. Eddy currents are an important effect; a much earlier coilgun found a 24% reduction in velocity due to eddy currents in a brass firing tube. Since then, we have only used non-conductive firing tubes such as the plastic ones in this coilgun. Barry's Coilgun (1)

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Barry's Coilgun Design you can analyze coilguns without building them and study effects that you can't build, by using finite element analysis (FEA) software and simulate your coilgun. Finite Element Magnetics

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Barry's Coilgun Design FEM Models - Hollow Cylinder Projectile ＊ The graphs for seven different projectiles are practically on top of one another. Therefore, the force per unit of mass does not depend on the inside radius of a hollow cylinder of iron. Finite Element Magnetics

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Barry's Coilgun Design FEM Models - Projectile Length Finite Element Magnetics ＊ These results confirm the rule-of-thumb that projectiles should be about the same length as the coil.

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Barry's Coilgun Design FEM Models - Iron at Coil Entry ＊ There is no dependence on work and washer thickness. The total kinetic energy is practically the same for every washer! Finite Element Magnetics Thickness (mm) Work (Joules)